Chlamydocin is a highly cytostatic cyclic tetrapeptide produced by cultures of Diheterospora Chlamydosporia. At 0.3 x 10 to the 9th power g/ml, it inhibits the growth of P-815 mouse mastocytoma cells in vitro more efficiently than actinomycin D, amethopterin, colchicine or vincristine. Chlamydocin has low mammalian toxicity because it is rapidly inactivated in vivo in the blood. For this reason the oncostatic activity by injection is low although when administered by interarterial perfusion the oncostatic activity is substantial. Preliminary mechanism of action studies carried out here suggest that chlamydocin inhibits a process which is essential to the induction of the synthesis of new proteins rather than the process of protein synthesis itself. The objective of this research is to synthesize chlamydocin, study its mechanism of inactivation and prepare new analogs with improved half-lives and specificities. Using tritiated chlamydocin the mechanism of action will be determined and attempts will be made to identify chlamydocins receptor in PHA/TPA activated bovine lymphocytes.